The mitochondrial genomes of the mesozoansIntoshia linei,Dicyemasp. andDicyema japonicum

Abstract

The Dicyemida and Orthonectida are two groups of tiny, simple, vermiform parasites that have historically been united in a group named the Mesozoa. Both Dicyemida and Orthonectida have just two cell layers and appear to lack any defined tissues. They were initially thought to be evolutionary intermediates between protozoans and metazoans but more recent analyses indicate that they are protostomian metazoans that have undergone secondary simplification from a complex ancestor. Here we describe the first almost complete mitochondrial genome sequence from an orthonectid, Intoshia linei, and describe nine and eight mitochondrial protein-coding genes from Dicyema sp. and Dicyema japonicum, respectively. The 14 247 base pair long I. linei sequence has typical metazoan gene content, but is exceptionally AT-rich, and has a unique gene order. The data we have analysed from the Dicyemida provide very limited support for the suggestion that dicyemid mitochondrial genes are found on discrete mini-circles, as opposed to the large circular mitochondrial genomes that are typical of the Metazoa. The cox1 gene from dicyemid species has a series of conserved, in-frame deletions that is unique to this lineage. Using cox1 genes from across the genus Dicyema, we report the first internal phylogeny of this group.

Dicyemida and Orthonectida: Two Stories of Body Plan Simplification

“…Protein identity between mitochondrial predictions for Dicyema sp. and the earlier published D. japonicum (Robertson et al, 2018) varies from 39% (nad2) to 75% (cox1). The majority of mitochondrial genes can also be found in the transcriptomic data, except for atp6 and nad5 .…”

“…The following gene sequences were found: cox1-3 , cob , nad1-5 , atp6 , rrnL , rrnS , trnH , trnI , trnK , trnL1 , trnN , trnP , trnQ , trnR , trnS2 , and trnY (Figure 6). In earlier studies the dicyemid mitochondrial contigs were found to carry either one protein coding gene (Watanabe et al, 1999) or a protein coding gene and a tRNA gene (Robertson et al, 2018). We found one circle that contains two genes – cox2 and rrnS , and three circles that contain two tRNA genes each.…”

“…(Armstrong et al, 2000; Gibson et al, 2007) and sucking lice (Shao et al, 2009). Notably, the mitochondrial DNAs from orthonectids Intoshia linei , Intoshia variabili , and Rhopalura ophiocomae retain typical structure for metazoans and encode the full set of mitochondrial genes on a single circular molecule (Robertson et al, 2018; Bondarenko et al, 2019). The reason why the mitochondrial Dicyema spp.…”